It’s hard to say beyond anecdotal impressions. There are rankings out there of average breast size by country, perhaps drawn from brassiere sales numbers, but I’ve never seen one which disambiguates cup size (itself an incomplete indicator) from overall BMI. An obese white European woman may have a larger chest on average than African women of a healthy weight, but it’s not clear which example theater audiences would find the most attractive on the basis of their bust.
That’s my juvenile speculation for the day.